A reverse-blocking IGBT (insulated gate bipolar transistor) can be realized by adding minor changes to the structure of a standard IGBT so as to render the device capable of withstanding reverse voltage. Reverse-blocking capability is needed in various applications, such as current source inverters, resonant circuits, bidirectional switches, matrix converters, etc.
What differentiates a reverse-blocking IGBT from a forward-only blocking IGBT is that the p-n junction at the die backside is protected from the diced edges of the die by a p-type region. The p-type region is conventionally formed by a very deep dopant diffusion process around the perimeter of the die (chip). The deeply diffused region encircles the die perimeter and occupies the full thickness of the die. In the reverse-blocking condition, the backside p-n junction stops the current. When the backside p-n junction is reverse biased, equipotential lines in the space charge region fail to reach the diced edges of the die and therefore induce no leakage along the unprotected diced edges. The collector potential can be transferred to a reverse blocking termination at the opposite surface of the die directly by the p-type region across the entire depth of the die.
Forming a p-type region around the perimeter of a reverse-blocking IGBT and across the entire thickness of the die proves difficult via conventional deep dopant diffusion processes. A diffusing species such as Aluminum which has fast diffusion in semiconductor materials such as Silicon do not have high active doping levels. Moreover, a high thermal budget is required, further limiting the integration options. Also, significant lateral area of the die is consumed by the deep dopant diffusion process, which increases the die size and can be only partially offset by beginning the diffusion process in the kerf area i.e. the region of a semiconductor wafer that is cut e.g. by a saw blade or laser so as to singulate (physically separate) semiconductor dies from one another.